The present invention relates to a switching apparatus for use in a digital communications system having one standby channel for N regular channels (1:N, where N is a positive integer).
Such a system having a 1:N channel configuration is used for improvement of system reliability. When one of the N regular channels is faulty, the system switches that faulty channel over to the standby channel. For further details of such a system, reference is made to a paper entitled "A 6 GHz RADIO FOR TELEPHONY APPLICATIONS," by E. Takeuchi et al., 1976 International Conference on Communications, Vol. II, June 14-16, published by the IEEE.
In this switching operation, a switchover is required without a data hit, i.e. without loss of data, which may cause serious problems such as a collapse in frame synchronization, which affects the synchronized state of the communication system as a whole. This hitless switching operation, however, is practically impossible if a fault exists in a radio-linked section because an enormous memory capacity is required for storing data corresponding to the time to detect the fault and complete the protection switching. While it may thus be impractical to avoid all loss of data, the elimination of those data hits that have resulted from the maintenance actions themselves have been given consideration. This means that in the maintenance actions a transmitting signal of a regular channel would be transmitted to the standby channel by activating a transmitting switch in the regular channel, and a receiving switch at the receiving end would switch from the regular channel to the standby channel or vice versa, all without any data hit.
The receiving switchover apparatus generally comprises relays having a high reliability and fail-safe function so as not to shut down the outputs of both the regular and standby channels even during the power source breakdowns. In this case, if the non-bridge type relays are used for both channels, there occurs a period during which the output becomes completely shut off because of the different operating time for the two relays, thereby causing disruptions for the succeeding equipment as discussed before. Accordingly, the use of a bridge type relay can eliminate the period during which the signals become shut off.
However, there still remain difficulties with a switching apparatus employing the bridge type relay since the bipolar code format is used for relaying signals between radio-linked section equipment and multiplexer/demultiplexer equipment and the output polarities of the two channels may become different from each other even though the signals are identical. In other words, even with the bridge type relay, if the codes of the two signals of the regular and standby channels are of opposite polarities, they may cancel each other. Accordingly, it is necessary to control the polarity of the bipolar codes for both regular and standby channels in order to effectively achieve a hitless switching operation. This at the same time requires the adjustment of the propagation time lags for both channels to achieve synchronization.